Computationele en conceptuele dichtheidsfunctionaaltheorie voor elektronische resonanties

Electronic resonances are metastable electronic states subject to autodetachment, that is, spontaneous loss of an electron. They exhibit unique reactions, for instance dissociative electron attachment (DEA), a process in which attachment of an electron to a stable neutral molecule results in the formation of an anionic resonance, a so-called temporary anion, followed by bond rupture. DEA is ubiquitous in nature and has great potential for industrial applications, since it offers enhanced control over chemical reactions as compared to traditional heat-driven chemistry. Examples include, but are not limited to, plasmonic catalysis on metal surfaces, and fabrication of three-dimensional nanostructures by focussed electron beam induced deposition of metals. Furthermore, the discovery of DEA as one of the processes driving radiation-induced DNA damage is regarded as a major breakthrough for research into cancer treatment via radiotherapy. While it is clear that DEA and electronic resonances in general deserve and receive much attention, their theoretical study remains challenging. Methods for the theoretical description and computational investigation of bound states in polyatomic molecules and their reactions are well established. Yet, the possibility of autodetachment in electronic resonances provides an obstacle, preventing application of the bound-state methods to these states. Numerous efforts have been reported so far, ranging from scattering theory over modifications to the conventionally used Hermitian electronic structure methods, to non-Hermitian complex-variable techniques that describe resonances in terms of complex-valued energies. Even though these methods have resulted in great advancements, analogs of density functional theory and other density-based conceptual methods frequently used for studying chemical reactivity and bonding of bound states are still largely missing for electronic resonance states. Yet, such methods would enable thorough characterization of their electronic structure at reasonable computational cost. In this context, this thesis focuses on the development and application of new density functional theory methods suitable for describing electronic resonance states. Chapter 1 outlines the background of electronic resonances and DEA as resulting from electron-molecule collisions, stating the motivations and objectives of this thesis in more detail. This is followed by an in-depth review of the theoretical methods used in this work, given in Chapter 2. The first technique for describing electronic resonances employed here is the charge stabilization and extrapolation method. It has often been used before for calculating the energies of temporary anions. In Chapter 3, for the first time its combination with properties from conceptual density functional theory and quantum theory of atoms in molecules is presented, via its extension to nuclear Fukui functions, atom-condensed electronic Fukui functions and bond critical points. The resulting procedure is applied to the temporary anions of chlorinated ethene derivatives, regarded as model systems for chlorinated unsaturated organic compounds ubject to DEA. Chapter 4 presents a further extension to spatial functions. More specifically, the electronic Fukui function and the electron localization function are calculated by extrapolation, because these contain more information than the quantities considered before. The same temporary anions of chlorinated ethene derivatives are considered. Seeking to enable prediction of the DEA process, Berlin's binding function is calculated as well, because from this function, one can extract knowledge about how changes in the electron density influence the molecular bonds. Its combination with the electron localization function results in a promising method not only allowing for prediction of DEA but also offering further insights into the characteristics of the electronic state leading to dissociation. Moving to a different kind of systems as a further application of the developed extensions to the charge stabilization method, Chapter 5 describes the investigation of the anions of perfluorobenzene (C6F6), perfluorotetrahedrane (C4F4), perfluorocubane (C8F8) and perfluoroadamantane (C10F16). They have been characterized as correlation-bound, i.e. they are unbound at the Hartree-Fock level of theory because this method lacks a description of correlation. Furthermore, the latter three compounds have been hypothesized to host the excess electron in the center of the cage-like structure formed by the carbon framework. First, the performance of density functional and coupled-cluster methods is assessed for the calculation of electron affinities of these molecules. Subsequently, the anionic electronic states of the 'cage' molecules are characterized by employing the electronic Fukui function and the electron localization function. Turning to complex-variable non-Hermitian methods, the use of complex absorbing potentials for Kohn-Sham density functional theory (CAP-DFT) is presented. Chapter 6 contains a detailed overview of the specific implementations carried out within the scope of this work. The resulting new method comprises a significant advancement, as it allows for employing not only the local density approximation but also the generalized gradient approximation and hybrid functionals. Next, its application for the calculation of complex resonance energies of the temporary anions of molecular nitrogen, formaldehyde, formic acid and ethene, as well as the corresponding results, are reported and discussed in Chapter 7. Finally, Chapter 8 presents the general conclusions of the thesis, summarizing how it contributes to the ongoing advancements in the field of theoretical and computational methods for electronic resonances by extension of the charge stabilization technique and development of new CAP-DFT methods. Given that these latter methods comprise a very recent development, this chapter also provides an overview of how they can be improved. Additionally, it gives an outlook to further applications and developments the methods developed in this work enable, indicating that there is still a wealth of potential chemical processes and computational techniques worth exploring.